Optical communication systems use light emitting devices to generate light and serve as the source of a light beam, and use a light detector for detecting the light and converting the light to an electrical signal. To minimize the size of the device, both the light emitter and light detector are preferably semiconductor devices, such as light emitting diodes and photodiodes. In order to further reduce the size of the device, and to allow the emitter and detector be incorporated with circuitry for operating the device, it is desirable to be able to form integrated devices having both the light emitter and the light detector on a single substrate.
One problem in integrating a semiconductor light emitter and light detector on a single substrate results from the fact that the requirements for the semiconductor active layer or layers for each of these devices is different to achieve optimum operating characteristics for each. Also, if the same layer is used as the active layer for both the light emitter and the light detector, during the operation of the device, the emitter heats more than the detector since it operates at a higher current density. Heating causes the emitter wavelength to increase, and the emitted light is no longer efficiently absorbed by the detector. Therefore, it has been the practice to deposit on a substrate a set of layers of materials suitable for a light emitting diode, and a second set of layers of material suitable for the light detector. In some integrated devices, the different layers are deposited on opposite sides of a substrate. However, for integrated circuits, it is desirable to form the layers on the same side of a substrate. To form different layers for the two devices on the same side of a substrate becomes complex. Therefore, it would be desirable to be able to form both devices from the same layers so as to simplify making the integrated circuit having both devices therein.